Table 1 Formation mechanisms reported for some metastable TiO₂ polymorphs.

Anatase

Smaller crystal sizes and lower pressure⁴⁵.

Anatase → Rutile

Growth post-nucleation to a large crystal size, reconstructive process³

Anatase → Baddeleyite

Small crystallite sizes under high pressures⁴⁵

Anatase → α-PbO₂

Large nanocrystals to macroscopic single crystals under high pressures⁴⁵

Anatase → Brookite

Controlled size in the tens of nanometers regime and the presence of surfactants^10,46,47.Twinning on anatase {112} facets⁴⁸.

Brookite

Helper ions such as NaOH¹⁷, C₂H₂O₄ + polymer²⁴, pH during hydrothermal reaction⁸, or Cl^- ions²⁵ during either solution or hydrothermal synthesis. Intermediate crystallite sizes¹⁰.